Contains all the DXF header vars in a dict like object.
For explanation of DXF header vars and their content see the DXF
specifications from Autodesk. Header var content are basic Python types
like string, int, and float as simple types and tuples of float values
for 2D- and 3D points.

Multiple 2D- or 3D vertices connected by lines. The DXF entity POLYLINE
is also used to define Polyfaces and Polymeshes, dxfgrabber defines
separated classes for this entities see: Polyface and
Polymesh.

The bulge is the tangent of one fourth the included angle for an arc
segment, made negative if the arc goes clockwise from the start point to
the endpoint. A bulge of 0 indicates a straight segment, and a bulge of 1
is a semicircle. If you have questions ask Autodesk.

Layers are nothing special, they are just another attribute of the DXF entity, dxfgrabber stores the layer as a
simple string. The DXF entitiy can inherit some attributes from the layer: color, linetype

To get the real value of an attribute value == BYLAYER, get the layer definition: